1. Field of the Invention
The present invention relates to a cable monitoring system capable of monitoring failure (disconnection and degradation) of a plurality of cables connecting a power supply and a welding gun for each cable without performing high voltage measurement and relates to a monitoring method of the same.
2. Description of the Related Art
When a cable supplying welding current to resistance welding equipment is suddenly disconnected while a production line is in operation, the cable needs to be changed after all the production line is shut down. The shutdown generally took a long time and significantly reduced productivity thereof. To prevent such a problem, cable monitoring systems have been invented and utilized.
One of the aforementioned cable monitoring systems monitors variation in impedance due to disconnection and degradation of a secondary conductor of the resistance welding equipment by means of welding current (see Japanese Utility Model Examined Publication No. 54-37780).
In recent years, a welding transformer and a welding gun, which were separated, are integrated and are generally carried as a so-called transgun by a welding robot for spot welding. Such a transgun does not include a long secondary cable connecting the welding transformer and the welding gun. Disconnection therefore occurs in a flexible primary cable which extends from a thyristor of a resistance welding controller provided on a power supply's side to the welding transformer integrated with the welding gun along a robot arm. This primary cable includes, for example, two power lines (cable cores) forming a round-trip electric path, a ground line, and control lines. These lines are coated with an insulating material, The control lines are connected to a servomotor or encoder for pressing the gun, various detectors, a valve for pressing the gun, and the like. The primary cable generally has a diameter of about 20 to 30 mm. This primary cable is repeatedly bent by an action of the robot arm during the operation of the production line and moreover causes self heating because of energization for welding. Copper lines are thus oxidized and thinned causing degradation. Accordingly, the primary cable is easily disconnected.
Furthermore, high voltage of 200 or 400 V from the power supply is directly applied to the primary cable. The voltage applied to the primary cable is high as described above compared to secondary voltage applied to the secondary cable, which is as low as 10 to 20 V. Accordingly, disconnection of the primary cable is more dangerous than that of the secondary cable. In measurement of resistance of the primary cable, to which high voltage is applied, high voltage is applied to a control line and detectors used for the measurement. Touching the control line is therefore dangerous, and the detectors are susceptible to damage.
To prevent disconnection of the primary cable from causing shutdown of the production line for a long time, a monitoring system monitoring failure of the primary cable has been proposed (see Japanese Utility Model Examined Publication No. 7-53822).
However, the monitoring system of the Japanese Utility Model Examined Publication No. 7-53822 has a structure in which, in measurement of the resistance of the primary cable at high voltage which is connected to the primary side of the transformer for resistance welding equipment, resistance between parallel lines of the primary cable is detected and disconnection/degradation is judged. Accordingly, it is impossible to detect resistance of each line and especially a necessary portion of the primary cable. There is a room for improvement in increasing monitoring accuracy. Moreover, this monitoring system is still in a situation where touching the control lines is dangerous and the detectors are susceptible to damage, similar to the monitoring system of the Japanese Utility Model Examined Publication No. 54-37780, since high voltage is applied to the control lines and detectors.
Moreover, as a monitoring system which prevents the aforementioned danger and eliminates the influence on peripheral devices, systems of Japanese Patent Unexamined Publications No. 59-159279 and No. 2001-232481 are disclosed.